Vibrating beam accelerometers are generally known in the art. Exemplary accelerometers and/or component parts such as force sensors for use with accelerometers are disclosed in U.S. Pat. Nos. 5,339,698, 5,501,103, 5,456,110, 5,450,762, 5,331,242, 5,367,217, and 5,456,111, the disclosures of which are expressly incorporated herein by reference.
A typical vibrating beam accelerometer can be etched from a silicon wafer using micromachining techniques which are generally known. The vibrating beam or beams of such accelerometers are used to form one or more resonators which control the frequency of one or more oscillator circuits. The vibrating beam or beams are generally connected between a frame and one or more proof masses and are configured so that an acceleration results in a tension or compression force along the beam or beams. Accordingly, changes in the resonant frequency of the beams occur and exemplary signals from the oscillators are then frequency modulated which indicates acceleration.
For additional background material on accelerometers, and in particular vibrating beam accelerometers, the reader is referred to a text by Anthony Lawrence entitled, Modern Inertial Technology-Navigation, Guidance, and Control, the disclosure of which is expressly incorporated herein by reference.
Some accelerometers can be formed in crystalline quartz. Such quartz possesses piezoelectric properties which can be utilized in connection with one or more vibrating beams to measure acceleration. Unlike crystalline quartz however, silicon is not piezoelectric. Accordingly, piezoelectric drive cannot be used or incorporated into a silicon-based system for measuring acceleration. One practical drive method suitable for use with silicon-based systems is electromagnetic drive. For electromagnetic drive, the vibrating beam or beams are placed in a magnetic field. Electrical current passed over or through the beam or beams exerts a force on the beams while the motion of the beam or beams in the magnetic field generates an electrical voltage. The resistivity of silicon, however, makes it impractical to use the conductivity of silicon to conduct the appropriate electrical current. One past solution has been to form or provide a layer of conductive material having a sufficiently low resistance over the beam or beams. An exemplary material is gold which can be readily patterned to have separate conductive layers on the different beams of the accelerometer. U.S. Pat. No. 5,501,103 incorporated by reference above describes such solutions. One particular drive circuit configuration requires electrical leads, in addition to those on the force sensing beams, between the proof mass and the frame. Separate silicon beams or struts having a metal disposed over an oxide have been used for this purpose.
One problem associated with the use of a metal layer over the vibrating beams or struts is that the metal material undergoes irreversible changes with temperature variations. As a result, changes in frequency which are not a true indication of the acceleration can be experienced by the proof mass. The exemplary gold material mentioned above exhibits this problem. While other metals and combinations of metals have been tried, none have resulted in sufficiently stable frequency over operating temperature ranges.
FIG. 1 shows a silicon micromachined vibrating beam accelerometer generally at 20. The accelerometer comprises a frame 22 and a first proof mass 24. Proof mass 24 includes a mounted end 26 and a distal end 28 away from or opposite mounted end 26. A flexure 30 is provided and extends between mounted end 26 and frame 22. As used in the context of this document, "flexure" will be understood to mean one or more flexure portions which are joined with a proof mass. Flexure 30 defines a hinge axis HA.sub.1 about which proof mass 24 can be moved in relation to an acceleration experienced by accelerometer 20 along an input or sensitive axis which is generally into the plane of the page upon which FIG. 1 appears. A vibrating beam assembly 32 is connected between frame 22 and proof mass 24. Assembly 32 includes a pair of vibratable beams 34, 36. A strut assembly 38 is provided and is connected between frame 22 and proof mass 24. Strut assembly 38 includes individual struts 40, 42.
In the illustrated example, a second proof mass 44 is provided and includes a mounted end 46 and a distal end 48 away from or opposite mounted end 46. A flexure 50 is provided and is connected between mounted end 46 and frame 22. Flexure 50 defines a hinge axis HA.sub.2 about which second proof mass 44 can be moved in relation to an experienced acceleration. A vibrating beam assembly 52 is provided and is connected between frame 22 and proof mass 44. Vibrating beam assembly 52 includes individual vibratable beams 54, 56. A strut assembly 58 is provided and connected between frame 22 and proof mass 44. Strut assembly 58 includes individual struts 60, 62.
Accelerometer 20 is etched from a wafer of silicon crystal with surfaces disposed in the 1,0,0 crystal planes. The accelerometer in practice is mounted directly or indirectly to a vehicle the acceleration of which is to be measured. Frame 22 and proof masses 24, 44 typically have thicknesses (into the plane of the page upon which FIG. 1 appears) which are generally comparable to the thickness of a silicon wafer, i.e., typically around 400 to 525 microns. Flexures 30, 50 have respective transition areas 29, 31, and 49, 51 which extend toward a central portion of each flexure which has a thickness of around 20 microns. In the illustrated example, vibrating beam assemblies 32, 52 comprise double ended tuning forks with respective end parts 35, 55 providing for good mechanical coupling of the vibrating beams.
FIG. 2 shows electrically conductive structure disposed over vibrating beam assemblies 32, 52, proof masses 24, 44, and strut assemblies 38, 58. The conductive material defines first and second conductive paths 64, 66 which extend between respective pairs of bond pads 68, 70 and 72, 74. Third and fourth conductive paths 76, 78 are provided and extend over vibrating beam assembly 52, proof mass 44, and strut assembly 58 as shown. Conductive paths 76, 78 extend between respective pairs of bond pads 80, 82, and 84, 86. An exemplary conductive material comprising the conductive structure defining paths 64, 66, 76, and 78 is gold which can be provided to a thickness of around 0.5 microns and which can be separated from the underlying silicon by a layer of silicon oxide which is typically 0.5 microns thick.
Vibrating beam assemblies 32, 52 are arranged so that an acceleration causes a tension force on one of the assemblies and a compression force on the other of the assemblies. A difference in frequencies between the vibrating beam assemblies provides an indication of acceleration. The electrically conductive structure defining the conductive paths, and in particular bond pads 68, 70, 72, 74, 80, 82, 84, and 86 are used to couple the vibrating beam assemblies with which each is associated to an external oscillator circuit. In the illustrated example in FIG. 2, first conductive path 64 is provided over one vibrating beam and one strut, and second conductive path 66 is provided over the other vibrating beam and the other strut. Similarly, third conductive path 76 is provided over one vibrating beam and one strut while fourth conductive path 78 is provided over the other vibrating beam and other strut. One of the vibrating beams for each proof mass is driven by a current, while motion of the other beam produces a voltage. Mechanical coupling between the vibrating beams of each beam assembly makes it possible to drive one beam and sense the motion of the other. In the illustrated example, hinge axes HA.sub.1 and HA.sub.2 are disposed on a common side of frame 22.
FIG. 3 shows an alternate accelerometer design. Like numerals from the above-described embodiment have been utilized with the suffix "a". In this example, proof mass 44a is rotated 180.degree. from that shown in FIGS. 1 and 2. Accordingly, the respective hinge axes of the proof masses are now disposed on different or opposite sides of frame 22a. This configuration has been found to have advantages which relate to near perpendicular alignment of the combined sensitive axes of the proof masses with the front and back surfaces of the accelerometer.
FIG. 4 shows an alternate accelerometer design. Like numerals from the above-described embodiment have been utilized with the suffix "b". In this example, there is no strut assembly connected to proof masses 24b, 44b and frame 22b. The conductive structure which forms conductive paths 64b, 76b over the vibrating beam assemblies of each proof mass are connected together at each respective proof mass end. The equivalent circuit of this configuration with the vibrating beams immersed in a magnetic field is a resistor, an inductor, and a capacitor connected in parallel.
FIGS. 5 and 6 show an accelerometer 20c which utilizes only one proof mass 24c and includes a pair of vibrating beam assemblies and strut assemblies.
FIG. 7 shows an embodiment which utilizes only one proof mass 24d and no strut assembly. Additionally, the conductive structure is connected together at the proof mass end as in FIG. 4.
The implementations described just above are susceptible to temperature-induced effects which can cause inaccuracies in the sensed acceleration of each proof mass. Specifically, temperature changes can cause the conductive structure defining each of the conductive paths over the accelerometer to expand and contract differently than silicon. This causes a change in length of the conductive structure which does not match the dimensional change in the remaining silicon structure. Accordingly, a force is generated on the vibrating beam assemblies, the strut assemblies (where incorporated into a particular design), and the associated proof mass or masses. Over extended temperature ranges, for example from -40.degree. C. to 100.degree. C., the metal or conductive structure undergoes irreversible changes so that even if the accelerometer is calibrated over several temperatures and corrections are made for temperature effects, the irreversible changes still cause errors. Similarity of the errors in the vibrating beam assemblies can result in less error in the frequency difference, but the error is still too great for some applications.
This invention arose out of concerns associated with providing accelerometers and methods of forming the accelerometers which are directed to solving problems associated with temperature changes and the effects such changes have on the corresponding structure of accelerometers.